Your breastfeeding diet should be very similar to your pregnancy diet... Well, besides all the french fries and ice cream. Go for lean protein, whole grains, and fruits and veggies, and be sure to get enough calcium and iron. Keep popping those prenatal vitamins every day to make up for any nutrients you might be missing. Keep your fish intake low — many varieties contain high levels of mercury, which can be bad for baby's brain development. The EPArecommends nursing moms limit their consumption to 12 ounces a week, and no more than 6 of canned white tuna. Steer completely clear of swordfish, shark, tilefish and king mackerel.
As for specific foods that will upset baby's stomach, it depends entirely on who you ask. Some doctors recommend avoiding high-fiber foods (beans and cruciferous veggies), acidic foods (tomato sauce and citrus), and spicy and pungent foods (garlic and onions). Others say that what you eat has absolutely no effect on baby's digestion or level of fussiness... after all, Indian mothers don't avoid spices, and Latin American women don't forgo beans. Every baby is different, so the best way to find out what foods upset yours is by eating just one of these foods at a time and observing whether baby seems especially fussy, gets a rash or has diarrhea.
One of the most common culprits is dairy — some babies have trouble digesting the protein in cow's milk, which can cause lots of gas and tears. If baby seems miserable, especially after a feeding, try cutting out dairy from your diet. You'll quickly find that a few months without cheese is a small price to pay for a few less minutes of listing to your little guy cry.